The high degree of amino acid sequence diversity among different HIV-1 isolates may be due in part to selection of resistant variants by neutralizing antibodies during the course of viral infection. Such selection would result in neutralization resistant mutant viruses able to escape surveillance by these antibodies. There are at least two neutralization determinants on the HIV-1 envelope. One, which is the most thoroughly investigated and defined, is the principal neutralization domain (PND) and is contained in a peptide in a disulfide cross-bridged loop (amino acids 296 to 331) on the surface of external envelope, gpl2O. The neutralizing antibodies elicited by the PND are virus type-specific. Another neutralization determinant(s) elicits antibodies that neutralize HIV-1 isolates with envelopes different in amino acid sequence and is likely to be conserved in amino acid sequence. The goal of this proposal is to investigate the mechanisms HIV-1 uses to escape neutralization by both of these types of antibodies. The specific aims are: 1) Starting with virus generated from infectious molecular clones of the IIIB, RF and MN HIV-1 isolates, select virus variants resistant to neutralization. The selecting antibodies will be: a) polyclonal and monoclonal antibodies specific for the PND and, b) HIV positive human neutralizing antisera that are devoid of PND binding antibodies; 2) The envelopes of these neutralization resistant or revertant viruses will be sequenced and the differences relative to the wild-type sequence will be located; 3) These mutations will be introduced into the wild-type envelope and those conferring the resistant phenotype will be identified; 4) Synthetic peptides of the identified regions will be made and used to generate antisera that will be assayed for neutralization activity.